Download Magnets and Magnetic Fields

Magnets and Magnetic Fields
Physics 1-2 Chapter 21
How did magnets get their name?
• First discovered about
3000 years ago
• Magnesia, Greece
• First naturally
occurring magnetic
rock, lodestone
• Made up of ironbased material
Where do magnetic fields come
from?
• All magnetic fields arise from electric
currents.
• In the case of permanent magnets in
ferromagnetic materials, the currents are
from unpaired electrons orbiting the
nucleus.
Magnetic Poles
• Magnets have a north pole and a south
pole
• Like charges:
– Opposite poles attract
– Like poles repel
• Can’t isolate south pole from north pole
– If magnet is cut, each piece will still have two
poles
What are magnetic domains?
Magnetic substances like iron, cobalt, and nickel are
composed of small areas where the groups of atoms are
aligned like the poles of a magnet. These regions are
called domains. All of the domains of a magnetic
substance tend to align themselves in the same direction
when placed in a magnetic field. These domains are
typically composed of billions of atoms.
Properties of Magnets
• Permanent ALL the time, called
permanent magnets
– Example: lodestones
• Classified as magnetically hard or soft
• Soft magnets:
– Example: iron
– Easily magnetized
– Loses its magnetic properties easily
Properties of Magnets
• Hard magnets:
– Example: cobalt, nickel
– More difficult to magnetize
– Don’t lose magnetism easily
• Soft magnets:
– Example: Iron in a staple or in a nail.
– Easy to magnetize
– Can be easily demagnetized by physical
shock or heating
Magnets exert magnetic forces on
each other
• Example:
– When magnet is lowered into bucket
of nails, it can pick up a chain of
nails
– Each nail is temporarily magnetized
by nail above it (exert magnetic
force on nail below it)
– Limit to how long chain of nails can
be
– The farther from the magnet
smaller magnetic force
Magnets exert magnetic forces on
each other
• Eventually, magnetic force not strong
enough to overcome force of gravity
bottom nails fall
Magnetic Fields
• Force exerted by magnets acts
at a distance
• Example:
– Move south pole of magnet toward another
south pole
– Magnet will move away
• Other forces act at a distance:
– Gravitational forces, force between electric
charges
Magnetic Fields
• ALL magnets produce a magnetic field
• Strength of magnetic field depends on:
– Material magnet made from
– How much object is magnetized
– How far from magnet.
• Magnetic field lines used to represent
magnetic field
– Like electric field lines represent electric field
Magnetic Field Lines
• Direction is defined as the direction that
the north pole of a compass will point at
that location. ( go from N to S )
• Form closed loops
• Field lines that are closer together
strong magnetic field
• Field line that are farther apart weak
magnetic field
• Magnetic field strongest near poles
Magnetic Field Lines
How do compasses work?
•
•
•
•
Analyze magnetic field’s direction
Compass: magnet on top of pivot
Aligns with Earth’s magnetic field
Can be used to determine direction as
Earth acts like a giant bar magnet
Earth’s Magnetic Field
• Earth’s magnetic poles not same as
geographic poles
• Geographic north pole (Canada) magnetic
south pole
• Geographic south pole (Antarctica)
magnetic north pole
• Poles of magnet named for geographic pole
they point to
– N: “north-seeking” pole
– S: “south-seeking” pole
Earth’s Magnetic Field
Earth’s Magnetic Field
• Source of magnetism is unknown
– Earth’s core made mostly of iron but too hot to have
magnetic properties
– Circulation of ions or electrons in liquid layer of
Earth’s core?
• Direction of Earth’s magnetism has changed
– 20 reversals in last 5 million years
– We are due for a reversal in the next few thousand
years!
• Aurora Borealis/Australis:
– Solar wind (charged particles emitted from sun) is
deflected by Earth’s magnetic field
Aurora Borealis- “Northern lights”
Aurora Australis
• Aurora Australis
Auroras
• Auroras are only visible at night in extreme
northern or southern latitudes.
• In cases of unusually high solar activity, the
auroras may be visible further south.
Ch21.2 : Electromagnetism
• There is a magnetic field associated with
any current (there is no magnetic field
without a current!)
• The magnetic field lines are co-encentric
circles around a straight wire. The field line
direction is given by the right hand rule.
Thumb points in the direction of the
current and fingers gripping the straight
wire point in the direction of the field.
Right hand rule (P770, hons P662):
Solenoid
• A long helically wound, insulated electric
wire. The magnetic field is concentrated
within the coil. It is further concentrated
when a ferromagnetic material is placed
inside the coil.
• Electromagnet: A magnet that consists of a
solenoid and a ferromagnetic core. The
magnetic field can be switched on and off
with the flow of electric current.
Solenoid
21.3 – Relationship between
current and magnetic field
• A charge moving (a current) through a
magnetic field experiences a force.
• F = B q v Sin θ
– F is force in Newtons
– B is magnetic field strength in Tesla, T
– q is charge in coulombs and,
– V is the velocity of the charge
– Θ is the angle between mag field and
motion direction.
Right hand rule shows direction of force
(P774, P650 Hons) on a positevely charged
particle USE LEFT HAND FOR ELECTRON
v
B
F
Homework :
• Hons. P679 Q1,2,3 (use Voltage to
calculate v first), 4, 5, 6. Draw picture!)
• Reg. P 775 Q1 to 5. Draw a picture!
Force on a current carrying
conductor
•F = B I l
–Where I is the current and l is the
length of the conductor
–There is therefore a force between
any 2 current carrying conductors
(note the demo)
–Do Q 1 – 5 page 778
–AND P779 1-5
• The force on a current carrying conductor
has uses in motors, moving coil meters
(any meter with a needle) and in any
device where electrical energy converts to
kinetic energy ……where motion is
produced.
An explanation of how a motor
works……
• http://youtu.be/fWyzPdyCAzU
Chapter 22 Induced current
• If a conductor is placed in a varying
magnetic field, a voltage is induced in the
conductor. (Faradays First Law)
• If the conductor can form a circuit, a
______ will flow.
• This induced voltage (emf) can happen in
one of the following ways:
• 1) Move the conductor into or out of the
field.
Inducing voltage (contd.)
• 2) Circuit is rotated in the field (angle
between conductor and field changes)
• 3) Change the intensity of the magnetic
field.
• Before we go and do numerical problems
based on this idea do some concept
problems on the previous topics:
• P769 Q1-4
• P779 Q1-5
Practical applications of
electromagnetic induction
• Motor: This is more of an application of
F = BIL . A current flowing through a
loop of wire between two magnet poles
experiences a force that causes
rotation (See and understand demo).
When the motor turns 180°, a
commutator (switch) changes the
direction of the current so that the
force is now changed 180°, and
rotation continues.
• A moving coil meter (galvanometer) is like
a motor without the commutator and it
also has a spring to return it to zero.
• Generator: Identical to motor in
construction BUT: the coil is forced to
rotate with the magnetic field by an
outside force (ex. A turbine) and the
induced voltage causes a current to flow.
• Speaker – works due to F = BIL.
• Sound is just a pattern of changing
pressure (vibration).
• A loudspeaker or headphone has a wire
coil placed in a permanent magnetic field.
Current passed through the coil causes
the coil to experience a ______ .
• If the current changes at the same rate as
the sound, the speaker coil and the
permanent magnet interact to vibrate the
coil at the same frequency as the desired
sound.
• Microphone – identical to the speaker
construction but the coil moves due to
sound vibrations, causing a current to be
induced in the coil. This current can than
be recorded or increased (amplified).
• A microphone is to a speaker as a
motor:generator is to a motor:generator
• A motor is to a generator as a
microphone:speaker is to a
microphone:speaker
• A guitar pickup works the same way as a
microphone.
Transformer
• Two coils of wire that have a magnetic material
between them. When an A.C. current flows in
the primary coil, a changing magnetic field is
produced in the magnetic material. This
changing magnetic field induces a changing
current in the secondary coil. Voltages may be
changed:
V2 = [N2 / N1] V1
Where 1 means primary, 2 secondary, N = # of
turns of wire
Do Q1-6 page 818 Hons. P722 Q54-57 and 59
• Pole transformer
• 38,000V to 240V
• Why are the 38kV
wires (on top) thinner
than the 240V wires?
• Pad transformer
Transformers (contd.)
• Transformers do not work with direct (constant)
current as a changing magnetic field is
necessary to induce a voltage in the secondary.
• Alternating current (changes direction 60 times
per second in US, 50 /sec outside Americas) is
necessary for a transformer to work.
• Depending on the ratio of N2 /N1 the voltage
may be stepped up or down by any amount.
• Efficiency can be as high as the high 90’s %
with some energy lost as heat in the coils and
due to eddy currents producing heat in the
magnetic core itself. Large transformers have
cooling systems to remove this heat which can
lead to failure.
• Thomas Edison built a network of power plants
in major cities producing DC current.
• They had to be very close to the power users
and the voltage produced was the same as
voltage consumed.
• Nicola Tesla emigrated to the US and was
asked to solve the problem of supplying power
to gold and silver mines in the West. Mines
(where power was used) were often miles from
fast-running rivers (where power was
produced). Wires had to be very thick ($$$) if
low voltage used (P=VI). Equipment in mines
dangerous if high voltage used in mines.
• Tesla’s Trillion dollar idea:
• Don’t use DC – use AC!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
• Use transformers at power plant to make
voltage very high before distribution (thin wires).
• Use transformers at mine to reduce the voltage
to safe, practical levels.
• This is what we do today.
• Power is distributed from the power plants at V
of the order of 106 Volts over great distances
over a grid. Voltage is stepped down
successively by transformers until your house
receives 240V or 120V.
• Interesting video of utility linemen:
• http://youtu.be/Oy81YP-q8R4
Answers to P818
•
•
•
•
•
•
1)
2)
3)
4)
5)
6)
1.2 X102 V
25 Turns
156:1
3.5 X 104 turns
2.6 X10 4 V
147 V
Lenz’s Law
• The induced voltage in a conductor in a
changing magnetic field, produces a current
that creates its own magnetic field which
opposes the original magnetic field (the induced
field opposes the change that produced it in the
first place)
• Induced currents in transformer cores and any
conductor in a changing magnetic field flow in
circles. The larger the area, the higher these
currents are. They are called “eddy currents”
• Sometimes these induced currents are
desirable … ex. eddy current braking etc.
• GBSPhysics163 - How does the Giant drop
work? (Nick)
• Eddy current brakes are used on
passenger trains a lot. The braking
depends on speed (F=Bqv) so a nice
smooth stop results.
Back e.m.f. and motors
• As motor speed increases, a voltage (emf) is
induced in the coil that opposes the original
current that made the motor rotate. The net
effect is to reduce the original current.
• Motors have a high starting current which
tapers off as speed of rotation increases.
Faraday’s first law
• Induced V (emf) = -NΔ (AB (CosΘ))/Δt
N is the number of turns of wire, A is area,
Θ is angle between B and the circuit or
wire. The current direction found by R.H.R. .
Could you figure the current direction?
Find current direction